The present invention relates to a new and improved gear measuring feeler.
In its more particular aspects the present invention relates to a new and improved gear measuring feeler intended for use in a gear measuring or testing machine and comprising a feeler rod which is pivotably mounted in a housing and supports at one of its ends a feeler probe and at the other end one of two relatively movable members or parts of a measuring system. The measuring system generates a magnetic field and supplies electrical signals, which are proportional to the feeler probe deflection, to a matching or accommodation circuit which transforms the electrical signals to standardized values.
In a gear measuring feeler of this type as known, for example, from German Patent Publication No. 2,364,918, published Aug. 7, 1975, and the cognate U.S. Pat. No. 3,943,633, granted Mar. 16, 1976, the electrical signals are inductively generated in response to the deflection of the feeler probe in the measuring system and converted to standardized values, so that, for example, a deflection of 1 .mu.m corresponds to a measured voltage of 1 mV at the output of the gear measuring feeler.
In the known gear measuring feeler the feeler rod supports an exchangeable feeler probe at one end and the measuring system at the other end. The feeler rod is horizontally pivotably mounted in the housing by means of a type of torsion rod or bar and comprises a bulge portion or bead which is loaded by leaf springs. Two screws are screwed into the housing in diametrically opposed positions and perpendicular to the horizontal pivot plane defined by the feeler rod. By means of these two screws the feeler rod is allowed some play, however, is prevented from pivoting in a vertical plane. Two resilient stops are arranged in diametrical opposition to each other in the housing and within the pivoting plane of the feeler rod. These stops prevent too large horizontal pivoting movements of the feeler rod and absorb horizontal impacts of the feeler rod. The measuring system in the known gear measuring feeler comprises a differential transformer, the coil system of which is fixedly connected to the feeler rod and the transformer core of which is fixedly connected to the housing. The measuring system further comprises an electric circuit associated with the coil system and accommodated in a capsule which is also fixedly connected to the feeler rod. The electrical circuit is connected to the output terminals of the gear measuring feeler by means of highly flexible conductors.
The coil system comprises a primary coil and two secondary coils which are arranged on both sides of the primary coil. Each of the coils has been corelessly wound and is arranged in an eddy current cylinder mounted at the feeler rod. The transformer core is mounted at a brass pin which, in turn, is mounted at each one of both of its ends at two respective housing flanges by means of three adjusting screws which are in angular offset relationship from each other by 120.degree..
The air gap formed between the transformer core and the coil system amounts to approximately 2/10 mm, and the deflection of the coil system in the measuring range amounts to .+-.150 .mu.m, corresponding to a total deflection path of the coil system of 300 .mu.m within the measuring range. No problems occur within the measuring range due to the relative movement of the transformer core and the coil system, however, disturbances may arise when the gear measuring feeler is moved so as to abut the stop, such as during coarse operating errors. While the gear measuring feeler as such is protected by means of a throw-out or release protection provided at the feeler probe, the coil system, however, may be clamped to or bind with the transformer core adjacent the limiting stops, which in the most frequent cases results in destruction of the coils and at the very least requires readjustment of the gear measuring feeler.
The manufacture of the differential transformer and the circuit associated therewith is highly expensive in terms of work and also very troublesome. The eddy current cylinder and the transformer core are made of iron which has been subjected to a precise annealing procedure. Additionally, the transformer core still must be gold-plated as a protection against corrosion. The winding of the coreless coils is also troublesome because the same comprise a copper wire having a diameter of only some hundredths of a millimeter which carries a two-layered insulation made of nylon or polyvinylchloride (PVC) which is melted thereon under the action of heat so as to bake the coils to each other to form an inherently rigid coil system. Also the adjustment of the differential transformer is expensive and laborious because the transformer core must be very precisely mechanically adjusted by means of the adjusting screws in order to prevent mechanical eccentricities. At the same time the brass pin which supports the transformer core must be fixedly clamped using the same adjusting screws.
The construction of the electric circuitry associated with the differential transformer also is associated with great difficulties. Since the differential transformer constitutes an inductive system in which a voltage must be transmitted from the primary coil to the secondary coil only an alternating-voltage can be used. Therefore, the electric circuit contains a blocking oscillator comprising two transistors and four resistors which are supplied by a d.c.-voltage. An alternating voltage of a frequency in the range of about 7000-9000 Hz is thus generated in the primary coil. The a.c.-voltage is transferred to the secondary coils via the transformer core. Depending upon the position of the secondary coils relative to the transformer core more or less voltage is induced in one or the other of the secondary coils. The measuring signal results from the difference of the voltages induced at the two secondary coils. The a.c.-voltage is rectified in rectifiers, smoothed in a filter and then is applied to the output terminals. In such known circuitry difficulties result by virtue of the requirement that the two transistors have to be as equal to one another as possible, i.e. should have operating or working points as close to one another as possible since otherwise they would generate rectangular or squarewave pulses of unequal width, so that the measuring result would be negatively affected. The use of paired transistors, however, requires precise measurement of the same, a process which is quite expensive. Furthermore, the measuring voltage which is obtained at the output terminals includes a remaining or residual carrier signal of 20 mV which cannot be eliminated. As long as the measuring signal is directly fed to a recording system containing mechanical attenuation or dampening, the remaining or residual portion of the carrier is not disadvantageously noticed. Presently, however, the measuring signals are conducted via A/D-converters and are further processed in a computer, in which case the residual portion of the carrier signal may result in incorrect results. Such could only be prevented by working either with considerably more expensive circuits or with a d.c.-voltage. For the reasons indicated hereinbefore the latter is impossible with the inductive measuring system of the known gear measuring feeler.
Furthermore, the measuring system in the prior art gear measuring feeler is supplied from a separate external current source comprising an integrated circuit. This expense is necessary because the blocking oscillator must be supplied with a constant current, so that it can correctly operate throughout the entire intended temperature range.
Additionally, the output of the measuring system of the known gear measuring feeler is a high-impedance or high-ohmic output. However, it is always of greater advantage when long conductors connected thereto would have a low-impedance line termination because in such case interfering stray pick-ups would be smaller.
Finally, the frequency response range in the measuring system of the prior art gear measuring feeler is relatively limited and only amounts to about 0.5 kHz. The carrier of the information in the measuring system has a frequency in the range of 7-9 kHz, some time-constants are present therein due to mechanical inertia and, finally, the filter is a low-frequency filter i.e. possesses a large inertia, which is needed in order to hold the residual portion of the carrier frequency signal as small as possible. Due to all these circumstances the frequency response range is no higher than about 0.5 kHz.